I dont have an exhaust fitted yet, well i do but not a performance one. It was knocking quite badly, so i put it up on the ramps had a look underneath and it was one of those rubbery things that hold the back box in place. I gave it a prod and it all came apart in my face. So after a full dictionary of chockingly swearing and splutering i went down to one of the local garages. He gave me a new rubbery thing free. I fitted it and it fixed the problem. Sooooo my suggestion is either check that or.... When you are under the car look at the exhaust, on mine it has the back box then goes straigt (i think) then bends to the right. That bend and the back box are close to variouse parts of the car. So you if you pad those areas you should be ok. But please remember not to pad the actual exhaust, or you may have a fire under your car.
paul